JP2003165736A - Method for manufacturing optical fiber preform and device and manufacturing optical fiber preform using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of stably manufacturing for a long period an optical fiber preform from which an optical fiber is manufactured by melt drawing without increasing the transmission loss of the optical fiber, and a device for manufacturing the optical fiber preform using it. <P>SOLUTION: The device for manufacturing the optical fiber preform is provided with a core tube 2 comprising a cylindrical fused silica, which takes in, dehydrates and sinters a porous material for optical fiber with a porous layer on which glass fine particles are accumulated in the radial direction of the periphery of a cylindrical starting member. The concentration of metal impurities contained in the core tube 2 is 25 ppm or less, and, if the inside diameter of the core tube 2 is designated as d and the thickness of the core tube 2 as t, d/4t is 10 or more. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an optical fiber preform by dehydrating and sintering a porous material for optical fiber to form an optical fiber preform, and an optical fiber preform producing apparatus using the same. Regarding

[0002]

2. Description of the Related Art As a method for manufacturing an optical fiber, a porous material for an optical fiber is formed, the porous material for an optical fiber is dehydrated and sintered to form an optical fiber base material, which is then melt-drawn. There is a method to obtain an optical fiber.

In order to form an optical fiber preform by dehydrating and sintering the porous material for optical fiber, the porous material for optical fiber is made of quartz glass provided in an optical fiber preform manufacturing apparatus. It is housed in a tube, and helium (He) and, if necessary, chlorine-based dehydrated gas are added to the core tube,
It is general to perform heat treatment at a high temperature of 1000 to 1600 ° C. in this inert gas atmosphere.

[0004]

By the way, when the heat treatment of about 100 porous materials for optical fibers is carried out by the above-mentioned method, the furnace core tube made of quartz glass is crystallized and becomes opaque. The phenomenon is called "devitrification"), and the mechanical strength of the core tube deteriorates. Therefore, it was necessary to replace the core tube after heating a predetermined number of porous materials for optical fibers.

The optical transmission loss of the optical fiber in the 1.31 μm wavelength band is usually required to be 0.34 dB / km or less. However, in order to improve productivity, as the size of the optical fiber preform is increased, the optical fiber preform obtained by melting and drawing the heat-treated optical fiber preform immediately after the replacement of the core tube has a wavelength of 1. 31
There was a problem that the transmission loss in the μm band was 0.34 dB / km or more. After the core tube was replaced, the optical fiber preform formed after several heat treatments was melt-drawn, and the transmission loss of the obtained optical fiber was measured. As shown in FIG. 0.34 dB / km
It became the following, and there was a problem of being stable.

It is considered that such a problem is caused by an increase in heat treatment temperature and heat treatment time of the optical fiber preform as the optical fiber preform becomes larger. When the heat treatment temperature and the heat treatment time are increased, the metal impurities contained in the furnace core tube made of quartz glass are likely to diffuse into the inert gas atmosphere. Then, the metal impurities diffused in the inert gas atmosphere adhere to the surface of the optical fiber preform, and the optical fiber obtained by melting and drawing the optical fiber preform with the metal impurities has a light transmission loss. To increase.

From the above, by performing the heat treatment of the optical fiber preform several times, the metal impurities contained in the quartz glass core tube adhere to the surface of the optical fiber preform to a certain extent, and It is considered that the light transmission loss becomes stable when the concentration of metal impurities in the active gas atmosphere decreases. Further, in order to increase the strength of the core tube and reduce the frequency of replacement of the core tube, the thickness of the core tube is increased,
It is considered that the concentration of metal impurities contained in the core tube is high.

The present invention has been made in view of the above circumstances, and stably manufactures an optical fiber preform that does not increase the transmission loss of an optical fiber obtained by fusion drawing of the optical fiber preform for a long period of time. An object of the present invention is to provide an optical fiber preform manufacturing method and an optical fiber preform manufacturing apparatus using the same.

[0009]

[Means for Solving the Problems] The above-mentioned problem is obtained by depositing glass fine particles in the radial direction of the outer peripheral portion of a cylindrical starting member to form a porous layer, thereby forming a cylindrical porous material for an optical fiber. In the manufacturing method of the optical fiber preform for producing, the optical fiber preform is housed in a furnace core tube made of quartz glass, the optical fiber porous material is sintered, and the optical fiber preform is produced. Assuming that the concentration of the metal impurities contained in the core tube is 25 ppm or less, the inner diameter of the core tube is d, and the thickness of the core tube is t, d / 4t is 10 or more. Solvable. Further, the above-mentioned problem is a cylindrical quartz for carrying out dehydration and sintering by accommodating a porous material for an optical fiber having a porous layer in which glass particles are deposited in a radial direction of an outer peripheral portion of a cylindrical starting member. An apparatus for manufacturing an optical fiber preform having a furnace core tube made of glass, wherein the concentration of metal impurities contained in the furnace core tube is 25 ppm or less, the inner diameter of the furnace core tube is d, and the thickness of the furnace core tube is If t, it can be solved by an apparatus for manufacturing an optical fiber preform having d / 4t of 10 or more.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
FIG. 1 is a schematic configuration diagram showing an example of an optical fiber preform manufacturing apparatus of the present invention. The manufacturing apparatus of the optical fiber preform of this example includes a cylindrical core tube 2 made of quartz glass for containing the optical fiber porous material 1 and an optical fiber porous material 1 housed in the core tube 2. Heater 3 for heat treatment
And a heating furnace 4 accommodating the heater 3, an inlet 5 for introducing an inert gas into the furnace core tube 2, and a support rod 6 for supporting the porous material 1 for an optical fiber. ing.

The concentration of metal impurities such as iron (Fe) and aluminum (Al) contained in the core tube 2 is preferably 25 ppm or less, more preferably 20 ppm or less. The concentration of metal impurities contained in the core tube 2 is 25
When the content exceeds ppm, the metal impurities diffused in the furnace core tube 2 due to heating easily adhere to the surface of the optical fiber preform, and the optical fiber preform formed by the heat treatment in the furnace core tube 2 is melt-drawn. The transmission loss of the obtained optical fiber at the wavelength of 1.31 μm increases.

The size of the furnace core tube 2 is not particularly limited and is appropriately set according to the size of the porous material 1 for optical fiber accommodated therein. When the thickness is t, d / 4t is preferably 10 or more, and more preferably 30 or less from the viewpoint of the strength of the core tube 2. Regarding the optical fiber preform manufacturing apparatus of this example, the amount of metal impurities contained in the core tube 2 and the core tube 2
As a result of investigating the relationship between the inner diameter d and the thickness t, the following relationship was found. Since the metal impurities diffused into the core tube 2, the range in which the metal impurity is diffused through the cross section perpendicular to the longitudinal direction of the core tube 2 can be represented as [pi] d ^2/4 (Equation 1). The total amount of metal impurities contained in the cross section perpendicular to the longitudinal direction of the core tube 2 is (π / 4) ×
It can be expressed as {(d + 2t) ² −d ² } (Formula 2). The influence of the metal impurities on the transmission loss of the optical fiber obtained by melting and drawing the optical fiber preform heat-treated in the core tube 2 simplifies (1) / (2) and d
It can be expressed as / 4t (formula 3).

When d / 4t is less than 10, the metal impurities diffused in the furnace core tube 2 due to heating are likely to adhere to the surface of the optical fiber preform, and are formed by heat treatment in the furnace core tube 2. The transmission loss at the wavelength of 1.31 μm of the optical fiber obtained by melting and drawing the optical fiber preform may increase.

The heater 3 is an electric heating device whose temperature can be adjusted by adjusting the voltage or current, and is 900 to 160.
The temperature can be adjusted to 0 ° C. The outer wall of the heating furnace 4 is formed of a heat insulating material, and the heating furnace 4 is filled with an inert gas. In the optical fiber preform manufacturing apparatus of this example, the inert gas in the heating furnace 4 is heated to a high temperature by the heater 3, and the heat of the inert gas heated to the high temperature heats the inert gas in the furnace core tube 2. The gas is heated to heat-treat the optical fiber porous material 1.

Further, an inert gas supply section (not shown) including a cylinder filled with an inert gas is connected to the inlet 5 via an inert gas supply pipe (not shown). As a result, the core tube 2 is always filled with the inert gas. Further, the support rod 6 has a cylindrical shape formed of quartz or the like, and can rotate about the central axis of the support material for the optical fiber porous material 1 by the power of a motor (not shown) or the like. ing.

A method of manufacturing the optical fiber preform of the present invention will be described. First, a cylindrical starting member made of quartz glass is prepared. Next, while rotating the starting member around its central axis, glass raw material gases such as SiCl ₄ and GeCl ₄ are supplied together with oxygen and hydrogen into the oxyhydrogen flame of the oxyhydrogen burner to form glass fine particles. Synthesize
If the oxyhydrogen burner is traversed parallel to the longitudinal direction of the starting member, glass particles are deposited in the radial direction of the rotating starting member to form a porous layer, and the cylindrical porous material for optical fibers 1 To get Next, the porous material 1 for an optical fiber is supported by the support rod 6 and is housed in the core tube 2. Then, while rotating the optical fiber porous material 1 around the central axis of the support rod 6 in an inert gas atmosphere,
Dehydrate at 1200 ° C for several hours, then 1400 to 1600
The optical fiber preform is obtained by sintering at several degrees Celsius for several hours.

According to the method for producing an optical fiber preform of the present invention, a substantially homogeneous optical fiber preform can be stably obtained in the heat treatment immediately after the replacement of the quartz glass core tube and the subsequent heat treatment. it can. Therefore, immediately after the replacement of the core tube, the transmission loss at the wavelength of 1.31 μm of the optical fiber obtained by melting and drawing the optical fiber preform formed by the heat treatment in the core tube does not increase. Further, the strength of the core tube is less likely to decrease, and the frequency of replacement of the core tube decreases.

A concrete example will be shown below with reference to FIG. 1 to clarify the effect of the present invention. Example First, a cylindrical starting member made of silica glass having an outer diameter of 30 mm and a length of 600 mm was prepared. Then, while rotating the starting member about its central axis, in the oxyhydrogen flame of the oxyhydrogen burner, SiCl ₄ ,
A glass source gas such as GeCl _{4 is} supplied together with oxygen and hydrogen to synthesize glass fine particles at the tip of the starting member, and the glass fine particles are deposited in the axial direction of the rotating starting member to form a porous layer. A cylindrical porous material 1 for an optical fiber having a diameter of 200 mm and a length of 1000 mm was obtained. Next, the inner diameter d formed of silica glass having a metal impurity concentration of 20 ppm, 25 ppm or 30 ppm has an inner diameter d of 200 mm and a thickness t.
Of 4 mm, 5 mm or 6 mm, and metal impurity concentration of 20 ppm, 25 ppm or 30 ppm
A core tube 2 having an inner diameter d of 300 mm and a thickness t of 6 mm, 8 mm or 10 mm was prepared from the quartz glass. Each of these core tubes 2 is mounted on an optical fiber preform manufacturing apparatus, and the optical fiber porous material 1 is heat-treated in the core tube 2 while being rotated by a support rod 6 to obtain an optical fiber preform. It was Next, after exchanging the various core tubes 2 for the first time, the optical fiber preform obtained by heat-treating the optical fiber porous material 1 was melt-drawn and an optical fiber was obtained.
The wavelength of the optical fiber thus obtained is 1.31 μm
The transmission loss of light in was measured, and the relationship between the metal impurity concentration and d / 4t and the transmission loss was investigated. The results are shown in Table 1 and FIG.

[0019]

[Table 1]

From the results shown in Table 1 and FIG. 1, the concentration of metal impurities contained in the core tube 2 is 25 ppm or less and d / 4t.
Is 10 or more, the wavelength of the optical fiber obtained by melting and drawing the optical fiber preform formed by heat treatment in the core tube 2 is 1.31 μm for the first time after replacement of the core tube 2.
It was confirmed that the transmission loss in the above was 0.34 dB / km or less.

[0021]

As described above, in the method for producing an optical fiber preform of the present invention, glass particles are deposited in the radial direction of the outer periphery of a cylindrical starting member to form a porous layer,
A cylindrical porous material for optical fiber is manufactured, the porous material for optical fiber is housed in a furnace core tube made of quartz glass, and the porous material for optical fiber is sintered to manufacture an optical fiber preform. In the method for producing an optical fiber preform, the concentration of metal impurities contained in the core tube is 25 ppm or less,
Further, assuming that the inner diameter of the core tube is d and the thickness of the core tube is t, d / 4t is 10 or more. Therefore, in the heat treatment immediately after the core tube is exchanged and in the subsequent heat treatment, an almost uniform optical fiber mother fiber is used. The material can be stably obtained. Therefore, immediately after the replacement of the core tube, the transmission loss at the wavelength of 1.31 μm of the optical fiber obtained by melting and drawing the optical fiber preform formed by the heat treatment in the core tube does not increase. Further, the manufacturing apparatus of the optical fiber preform of the present invention, in the radial direction of the outer peripheral portion of the cylindrical starting member, accommodates the optical fiber porous material having a porous layer deposited glass fine particles, dehydration, An apparatus for producing an optical fiber preform having a core tube made of cylindrical quartz glass for sintering, wherein the concentration of metal impurities contained in the core tube is 25 p.
pm or less, and assuming that the inner diameter of the core tube is d and the thickness of the core tube is t, d / 4t is 10 or more. Therefore, it is formed by heat treatment in the core tube immediately after replacement of the core tube. The transmission loss at the wavelength of 1.31 μm of the optical fiber obtained by melting and drawing the optical fiber preform does not increase. Further, the strength of the core tube is less likely to decrease, and the frequency of replacement of the core tube decreases.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of an optical fiber preform of the present invention.

FIG. 2 is a graph showing the relationship between the metal impurity concentration and d / 4t, and the transmission loss.

FIG. 3 shows an optical fiber preform formed by a conventional optical fiber preform manufacturing method, the number of times the optical fiber preform is processed after the replacement of the core tube, and the obtained optical fiber preform by melt drawing. It is a graph which shows the relationship with the transmission loss in wavelength 1.31 micrometer of the obtained optical fiber.

[Explanation of symbols]

1 ... Porous material for optical fiber, 2 ... Core tube, 3 ... Heater, 4 ... Heating furnace, 5 ... Inlet port, 6 ... Support rod

Claims (2)

[Claims] 1. A radial direction of an outer peripheral portion of a cylindrical starting member,
Glass microparticles are deposited to form a porous layer to manufacture a cylindrical porous material for optical fiber, and the porous material for optical fiber is housed in a furnace core tube made of quartz glass, In the method for producing an optical fiber preform, in which a high quality material is sintered to produce an optical fiber preform, the concentration of metal impurities contained in the furnace core tube is 25 ppm.
D / 4t is 10 or more, where d is the inner diameter of the core tube and t is the thickness of the core tube. 2. The radial direction of the outer peripheral portion of the cylindrical starting member,
A device for producing an optical fiber preform having a core tube made of cylindrical quartz glass for accommodating a porous material for an optical fiber having a porous layer on which glass fine particles are deposited and performing dehydration and sintering, An optical fiber having a concentration of metal impurities contained in the core tube of 25 ppm or less, d / 4t of 10 or more, where d is an inner diameter of the core tube and t is a thickness of the core tube. Base material manufacturing equipment.